Our goal is to understand the developmental biology of pancreatic beta-cell origin and fate as a function of age. Limited beta-cell regeneration may play a fundamental role in type 2 diabetes mellitus (T2DM), especially in the elderly. beta-cell mass is reduced in T2DM patients, which may impair beta-cell function. In the past ss-cell mass was considered to be highly dynamic. But our studies suggest that beta-cell regeneration is restricted in maturity. We therefore hypothesize that the age-related decline in beta-cell regeneration contributes to diabetes pathogenesis. Whereas self-renewal is the primary mechanism of beta-cell growth in young mice, beta-cell neogenesis might also occur under some circumstances. Surprisingly, beta-cell mass continues to expand in aged mice, despite severely reduced ss-cell turnover rates. We therefore hypothesize that ss-cell neogenesis could contribute to beta-cell mass expansion in the elderly. Thus, the following aims: 1.) Determine the lifespan and cell of origin of pancreatic beta cells in healthy aged mice. We will quantify cell turnover rates of aged beta-cells, and define the origin of new beta- cells in healthy aging. 2.) Determine the lifespan and cell of origin of pancreatic beta-cells in pathological aging. We will quantify beta-cell mass expansion of aged mice in response to regenerative stimuli, injury (partial pancreatectomy, pancreatic ductal ligation, streptozotocin mediated beta-cell loss) or diabetes therapies (exendin- 4, sitagliptin). We will then define the cell of origin of new beta-cells in pathological aging, carrying out lineage tracing studies of ss-cells combined with regenerative stimuli. Finally, we will test the hypothesis that the "replication refractory period" of beta-cell turnover limits cell cycle entry of aged beta cells. We will determine the number of cell divisions of beta-cells in aged mice in response to regenerative stimuli. In summary, these studies use innovative tools to precisely follow fate of aged beta-cells and their progenitors during regeneration.